[time-nuts] Standards for units
pvince at theiet.org
Tue Apr 3 05:34:59 EDT 2007
Now I thought the NTSC frame-rate was reduced so that the line
frequency was an exact sub-multiple (1/286) of the sound subcarrier
at 4.5 MHz, and hence sidebands of the color subcarrier were equally
positioned around the sound carrier. Using exactly 30 Hz frame rate
(15750 Hz line rate) would have necessitated increasing the sound
subcarrier by 4.5 KHz in order to minimise picture-on-sound
interference. My understanding is that whilst this was the most
obvious this to do, and the major television manufacturers had no
problem with that, one or two minor manufacturers of black-and-white
televisions objected vociferously that the new signal would be
incompatible with their old televisions, and so the FCC insisted the
sound carrier had to remain at 4.5 MHz - a decision everybody in
television has regretted ever since!
I thought I had learnt the above from the SMPTE NTSC specification
170M, but on re-reading that now, I can't find the reference :-( I'd
love to read that RCA document if it is available electronically.
Peter Vince (BBC Television, London)
>This precise sort of error reminds me of the error that RCA
>introduced into the frame rate for NTSC color when they discovered
>that the chosen color burst frequency was right on top of a diathermy
>machine or something silly like that. They bumped the frame rate down
>by exactly 0.1%, creating the 29.97 frames/second that
>cinematographers have grown to love to hate.
>There's a volume of the RCA Review that came out in 1953 that is all
>about the NTSC color system development process. It's fascinating
>reading. Available at your local university library.
>--David Forbes, Tucson, AZ
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